In the field of irrigation it is necessary to ensure that water is properly controlled and supplied to vegetation. Some plant matter suffers in an abundance of water and others in a lack of. To this end, it is important to be able to determine the water content of the supporting soil particularly in the regions proximate to the vegetation's major root structures.
Many examples of sensors are detailed in prior art and can be loosely group into the following:
Resistive/conductive sensors. U.S. Pat. No. 4,796,654 is an example. These type of sensors rely on the resistive/conductive property of moist soils and consist of at least two electrodes inserted into the soil and a means to measure the resistance/conductance between these electrodes. The resistive/conductive property of soils changes with its composition. Property such as salinity and acidity greatly affects resistive/conductive readings and for this type of sensor to be effective, a comparative method is required to calibrate the sensor to the soil. These types of sensors were therefore difficult to install and maintain.
Known dielectric sensors. U.S. Pat. No. 4,837,499 is an example. These type of sensors consist of a know material that is sandwiched between at least two plates electrodes. The material becomes the dielectric of the sensor. The sensor is place in contact with the soil to be measured and by contact the dielectric material becomes moist. The capacitance or resistance of the resultant sensor is measured. These types of sensors provided immunity to the composition of the soil but suffer from a limited useful life since the dielectric material degrades with time. Additionally these sensors suffer from slow response since the dielectric material takes time to reach the same moisture level as the surrounding soil.
Capacitive sensors. U.S. Pat. No. 5,859,536 is an example. These types of sensor consist of at least two electrodes separated by the soil to be measured. The capacitance of the resultant capacitor is measured. These sensors provide immunity to the composition of the soil, however, with these sensors it is difficult to determine the wetness at a particular depth. To achieve this the user has to excavate and bury the sensor at the desired depth. This causes the density of the soil to change and therefore the measured wetness is not a true representation of the original soil sample.
Other available sensors are generally not suitable for the layman and or require expensive measuring equipment.